1. Field of the Invention
The present invention relates to an improved floating input comparator and, more particularly, to a floating input comparator capable of mitigating the effects of parasitic capacitance.
2. Description of the Prior Art
Floating input comparators are useful in general applications where comparator analog inputs are not valid for a full detection cycle or where one comparator input must be held for several detection cycles. In form, comparators are high gain, uncompensated differential amplifiers, where input stage capacitances (referred to as parasitic capacitances) couple a portion of the associated gain back to the input.
One exemplary CMOS comparator is disclosed in U.S. Pat. No. 4,075,509 issued to T. P. Redfern on Feb. 21, 1978. Here, a novel mask geometry is provided for the gate electrode such that the total source/drain area under the gate remains the same regardless of the alignment of the mask within a certain predetermined tolerance. The effects of parasitic capacitance in the Redfern arrangement are minimized by the inclusion of a dummy transistor in the first stage amplifier.
The effects of parasitic capacitance may, in some cases, be compensated by the use of an array of switchable resistors, as disclosed in U.S. Pat. No. 4,301,419 issued to T. J. Calomiris on Nov. 17, 1981. While such an arrangement is useful in many cases, for example, to maintain a relatively constant Q and gain in a CMOS switched active filter, its use in association with a comparator circuit is ineffective.
Various other prior art methods exist for overcoming the problems associated with parasitic capacitance. For example, source-follower arrangements may be utilized to help negate the effects of the parasitics. Most notably, cascode arrangements are often employed. However, cascoding techniques merely reduce, not eliminate, the effects of parasitic capacitance.
The presence of any parasitic capacitance in a comparator circuit which utilizes floating inputs is especially harmful, since any stray capacitance may significantly alter the input values and cause the compartor to operate erroneously. Thus, there remains to be solved the problem of substantially eliminating the effects of parasitic capacitance on the operation of floating input comparators.